Transcript Specific immune response

Specific immune response
Biochemistry II
Lecture 13
2009 (J.S.)
Body's defence against microorganisms (bacteria, viruses,
fungi, cells or tissues of geneticaly distinct systems:
Non-specific mechanisms barriers – intact skin (stratum corneum) and mucous
membranes, the rinsing effect of glands secretion,
ciliary epithelial cells, acidity of stomach and vagina
secretion,
natural antibacterial substances – lysozyme, basic
polypeptides, interferons, chemo- and leukotactic
compounds,
non-induced phagocytosis of monocytes, tissue
macrophages (histiocytes, Kupffer cells, alveolar
macrophages in the lung, microglial cells), and
polymorphonuclear leukocytes.
Specific mechanisms – through the mediation of the immune
system – acquired humoral immunity
and cell-mediated immunity.
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Basal terms
Immunity is the body's ability to react on the presence of foreign
protein or heteropolysaccharide (an antigen) with useful
immune response as to eliminate antigens (microorganisms,
transplants, tumour cells) in order to retain the molecular
integrity and individuality of its own.
Immune response – the complex of reactions mediated through
lymphoreticular system that follow an invasion of the foreign
antigen into the body.
Lymphoreticular system (lymphoid organs or tissues) central, primary - thymus and equivalents of bursa of
Fabricius (present in birds),
peripheral, autonomic - spleen, lymphatic nodes, bone
marrow, tonsils, Peyer's patches (plaques,
small intestine), T and B lymphocytes.
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Antigens
Immunogens or complete antigens
are mostly macromolecular substances, which after they invade the
body, are recognized as foreign compounds by immunocompetent
cells, and which initiate a specific immune response, production of
antibodies.
Haptens are small organic molecules (such as short peptides, certain
drugs) that are recognized as foreign compounds but
don't initiate an immune response.
Haptens also can elicit antibodies, provided that they are attached to a
macromolecular carrier, which can be quite neutral from the
immunological point of view.
So the immunogen originates – a carrier with the haptenic determinant.
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Antigens from the chemical point of view:
proteins and polypeptides,
saccharidic components of glycolipids and glycoproteins,
bacterial heteropolysaccharides, peptidoglycans and
lipopolysaccharides,
some nucleic acids can act as immunogens, and
currently, some phospholipids are also mentioned.
The part of immunogen molecule that initiates the specific immune
response (it can be very small) is called
a specific antigenic determinant or epitope.
On the surfaces of native protein molecules, two types of
determinants are present – ether sequential (3 - 8 amino acid
residues) or conformational (up to 20 amino acid residues).
Saccharidic determinants are mostly short oligosaccharides
(1 - 5 monosaccharide units) at the non-reducing end.
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Lymphoid stem cells of the bone marrow
differentiate postnatally to
immature T lymphocytes
maturing in the thymus or provided by
thymus hormones in the periphery
immature B lymphocytes
maturing doesn't depend on thymus
clones of immunocompetent cells
with the fixed genetic information to recognize a sole specific antigen
contact with the specific antigen causes activation of cells of the particular clone
memory T cells
blastic transformation
memory B cells
blastic transformation
the clone of mature
T cytotoxic cells and
T helper cells
the clone of plasmocytes
producing specific
antibody (immunoglobulin)
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Immunoglobulin molecule (IgG)
Two light chains – variable domain and constant domain
Two heavy chain – variable domain and 3(-4) constant domains
VL
CL
CL
VL
VH
CH1
CH1
VH
Chains within globular
domains are stabilized
by disulfide bonds
CH2 CH2
Disulfide bonds
between chains
CH3
CH3
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Model of immunoglobulin G
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Variable domains of both heavy and light chains form at their NH2-ends
two coincident binding sites for the specific antigen determinant that
are quite variable from one antibody to another.
VL
VL
VH
VH
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Terciary structure of light chains
NH2
Variable domain VL
(3 hypervariable loops,
complementarity-determining regions)
Constant domain CL
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Antigen-binding site of an immunoglobulin
VL
VH
Side view
View from above in the direction
of molecule axis
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Treatment of intact IgG molecules with
the proteinase papain results in the
formation of three large fragments:
two Fab fragments („antigen binding“,
monovalent) and Fc fragment
("crystallisable", it can be easily isolated
in the crystalline form).
Treatment with proteinase pepsin
results in the formation of divalent
(Fab)2 fragment and Fc fragment.
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Functions of immunoglobulin domains
Variable domains VL a VH are responsible for the distinctive
function of immunoglobulins, forming together a binding site for
a specific antigenic determinant. Specifity of binding sites in high,
it depends on the amino acid sequence of hypervariable loops
(complementarity-determining regions, there are three in VL and
four in VH).
Each antigen-binding site can bind noncovalently one antigenic
determinant or one hapten. The strength of this interaction is
called affinity.
As a rule, binding sites exhibit high affinity for only a limited
number of similar determinants. With decreasing strength of
interactions, the number of such "cross-reacting" determinants
increases. Numerous determinants are bound very weakly,
however these weak interactions are not significant practically.
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Constant domains mediate biological functions called
effector functions:
Interaction of variable domains with the antigen initiate the process,
the result of which is antigen elimination.
Domains CL a CH1 are connected through disulfide bond. The change in
conformation evoked by the interaction with antigen induces
conformational changes of all remote constant domains. In the complement
cascade, CH1 domain binds the complement component C4b.
The hinge region joins both heavy chains. In the heavy chains of IgM is
the hinge substituted by special domains CH 2.
Domains CH2 of immunoglobulins IgG a IgM are binding sites for the first
complement component C1q or certain immunomodulating peptides.
Domains CH3 (in IgM CH4) enable together with domain CH2 cytotropic
reactions – binding to Fc-receptors of phagocytes and B or T cells,
which initiates readily either phagocytosis of immunocomplexes, or
formation of the complex with the cell exposing an antigen – a signal for
extinguishment of the cell.
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Five immunoglobulin main classes schematically
IgG
IgA
IgM (pentamer)
IgD
IgE
Secretory IgA
(dimer)
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IgG - immunoglobulins class G
Subclasses (isotypes) IgG 1 – 4
differ in the numbers and positions of disulfide bridges.
IgG 3 has up to 15 disulfide
bridges between its heavy chains
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IgA - immunoglobulins class A
Subclasses (isotypes) IgA 1 and IgA 2
Serum IgA
Secretory IgA (SIgA, dimer of serum IgA)
joining chain (J)
secretory component (SC)
Mr 70 000
It occurs in mucous secretion, where
it takes part in reactions of local immunity.
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IgM - immunoglobulins class M
Pentamers of identical basal subunits
joining chain a glycoprotein, Mr 15 000
Basal subunit of IgM – heavy chains
comprise four constant domains
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Classes of immunoglobulins - properties
IgG
IgA
IgM
IgD
IgE
Heavy chains
1, 2,  3, 4
1, 2
1, 2


Light chains
 or 
 or 
 or 
 or 
 or 
(22)5J
(22)5J
22
22
22
22
serum
Molecular
formula
22
22
22
22
secretory
(22)2JS
(22)2JS
Approx. Mr
Saccharides
Function
Serum
concentration
150 000
180 000
- 500 000
950 000
3%
8%
10 %
antibacterial antiviral and
and antiviral antibacterial
activity,
activity
complement
binding
~ 12 g/l
~ 3 g/l
antibacterial
and antiviral
activity,
complement
binding
~ 1,2 g/l
175 000
200 000
12 %
12 %
?
reagins
< 0,1 g/l
< 0,001 g/l
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Antigen-antibody interaction
The primary event is the formation of an antibody-antigen
complex (binding of the specific immunoglobulin to the corresponding
antigen). The binding of antigens to immunoglobulins usually results in
marked conformational changes.
Antigens are either soluble (colloid particles), or corpuscular (antigenic
determinants on the surface of cells or other insoluble particles).
Soluble Ag-Ab complexes are called immunocomplexes.
Two stages of the formation of immunocomplexes can be
distinguished: the binding itself that is relatively fast (formation of
non-covalent interactions, the most important of which are the
hydrophobic), and the complex transformation, which can take
longer time (the complex is stabilized through formation of more
interactions).
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Secondary processes associated with formation of Ag-Ab complexes
Immunoprecipitation. Immunoglobulin molecules include two
antigen-binding sites, so that they can cross-link soluble multivalent
antigens at certain limit concentration (and a proper concentration ratio
of both). These three-dimensional networks are insoluble and visible
as turbidities or precipitates. An excess of both antibody or antigen
inhibits precipitate formation.
Agglutination of cells or other particles is a similar process:
Immunoglobulins act as cross-links between antigenic determinants of
multivalent corpuscular antigens (cells, bacteria, generally
agglutinogens). Aggregates of particles (agglutinates) are easily
distinguishable from sediments of particles that are not agglutinated.
Cytotropic reactions. Fc receptors bind immunocomplexes, the result may be
either phagocytosis of the immunocomplex or (mediated by cytotoxic T cells)
extinguishment of the antigen-exposing cell.
Triggering of the complement cascade (the classical pathway of activation of
complement components) is a process that leads to the lysis of foreign target cells.
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Antigen cross-linking in immunoprecipitates (soluble antigens)
or agglutinates (corpuscular antigens)
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Diversity of antibodies is generated by gene rearrangements
Antigen-binding sites of molecules responsible for humoral immunity –
membrane-bound immunoglobulins of B cells (mIg), receptors of T-helper
cells (TCR), and antibodies produced by plasmocytes – exhibit an extreme
diversity. More than 108 different structures can be formed
(in cells responsible for the cellular immunity even about 1012 different
structures).
The sources of this diversity are both the combinatorial association of
short gene segments encoding variable-region genes and the high rate of
introduction of somatic mutations into the recombined genes.
The  light-chain gene (on chromosome 2) includes an array of 40 segments V that
encode the variable region, 5 segments J that encode the joining region (between the
VL and CL domains, and a single region that encodes the constant domain CL.
The arrangement of the  light-chain gene (on chromosome 22) is similar.
The heavy-chain gene (on chromosome 14) includes 51 segments V for the variable
regions, 27 segments D (diversity genes), 6 segments J for the joining region, and
further groups of segments encoding the heavy-chain constant domains CH1-4 of
distinct immunoglobulin classes.
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Activation of B lymphocytes – transformation to plasmocytes
The simplified diagram shows the TH cell-dependent B cell activation by the antigen:
Virgin B cell
DIFFERENTIATION
PROLIFERATION
pAg
Specific plasmocyte
Y
ANTIGEN
H
Ig
Memory B cell
IL-2,6, 4
Macrophage
Mature TH cell
ANTIGEN
Non-specific
phagocytosis
DIFFERENTIATION
PROLIFERATION
Memory TH cell
IL-1
Immature TH cell
IL-2
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The first specific activation signal evoking the transformation of B cells to
plasmocytes is the binding of specific antigens (e.g. molecules of soluble foreign
proteins on the membrane-bound
The antigen-presenting B lymphocyte
immunoglobulins (mIg) on the surfaces
of "virgin“ B cells. After internalization by
endocytosis, they are digested and
peptides with antigenic determinants –
processed antigens (pAg) – are
associated with class II MHC (major
histocompatibility complex) proteins,
move to the cell surface where they are
displayed.
Continuation of B cells transformation is
triggered by the second activation signal
- the binding of the T-cell receptor of a mature
helper T cell with corresponding specifity to the
antigen-presenting B cell, which results in the secretion
of cytokines. These cytokines bind to cytokine receptors expressed
on the surface of B cells, stimulating differentiation and antibody secretion.
(The immature helper T cell has been transformed to the mature cell by the interaction of its
receptor TCR with the same antigenic determinant that was presented in complex with II class
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MHC protein by a nonspecific-phagocytosing macrophage.)
Some details for thoughtful students:
The first activation signal – the binding of specific antigen to mIg of B cells:
On the surface of each B cell, there are about 10 5 membrane-bound monomeric mIgM with the same
specific binding sites. Those mIgMs are associated with dimeric proteins Ig-Ig, the cytoplasmic
domains of which includes sequences ITAM (immunoreceptor tyrosine-based activation motifs) with
tyrosyl residues (Y). The binding of a soluble antigen to mIgM evokes the activation of a tyrosine
proteinkinase (PK) and phosphorylation of tyrosyls Y triggers a cascade of following
phosphorylations.
Ag
B lymphocyte
mIgM
Ig- Ig
PK
ITAM
PK
phosphorylations
The protein phosphorylations result in the endocytosis of the Ag-mIgM complex, processing
of the antigen, and presentation of the antigenic determinant by means
of class II MHC protein onto the surface of the B cell.
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Specific receptors of T cells (TCR)
On the surface of each helper or cytotoxic T cell, there are numerous membrane receptors TCR.
These receptors consist of two chains ( a ) joined by disulfide bridge.
On the outer membrane side, each chain includes two domains (one variable and one constant) that
are homologous to the domains of immunoglobulin Fab fragments. Variable domains of both chains
form the monovalent binding site as in immunoglobulins. All binding sites in the particular clone of
T cells exhibit the same specifity.
T cell receptors recognize the corresponding antigenic determinants, but they cannot bind them,
unless these determinants are the components of complexes with MHC proteins on the
surface of other antigen-presenting cells.
binding site
T lymphocyte
binding site
 
T cell receptors are associated with the complex CD3 (cluster of differentiation 3) and
with either coreceptor CD4 (in helper T cells) or coreceptor CD8 (in cytotoxic T cells). 27
Association of T cell receptors (TCR) with the complex CD3, which consists of six
polypeptide chains (two heterodimers - a - and two polypeptides ). All chains form
conspicuous extracellular and cytosolic domains, all cytosolic domains of the complex CD3 include
sequences ITAM with tyrosyl residues (Y) that can be phosphorylated.
The chains of heterodimers , , and  are nearly the same as the chains Ig a Ig, which are
associated with membrane immunoglobulins M of B cells.
binding site of TCR
six chains of the complex CD3
heterodimers - a -
chains 
sequences ITAM
(immunoreceptor tyrosine-based
activation motifs)
Helper T cell receptors, in cooperation with coreceptors CD4, can bind only antigens
that are presented as complexes with class II MHC proteins, i.e. the antigens presented
by macrophages (in triggering of immature helper T cells transformation),
B cells (the second signal of B cells activation), and dendritic cells.
Cytotoxic T cell receptors, in cooperation with coreceptors CD8, bind determinants presented
by class I MHC proteins, that are on the surfaces of nearly all cell types.
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Maturation of helper T cells
triggered off by the antigen-presenting macrophage
Antigen being phagocytosed non-specifically by a macrophage is exposed as a processed antigen
determinant pAg on the cell surface by means of class II MHC protein. If the determinant is
recognized and bound to the receptor of immature helper T cell, the transformation of the T cell into the
mature T cell is initiated:
receptor for
interleukin 2
Macrophage
complex pAg-MHC II
CD4
Immature
helper T cell
Interleukin 2
TCR/CD3
PK
PK
Mature
helper T cell
initiation of
interleukin 2 secretion
The receptor of mature helper T cell binds to the antigen-presenting B cell (in the complex with
class II MHC protein - the consequence of the first activation signal). This interaction is the
second activation signal for transformation of the B cell into the plasmocyte.
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The second activation signal
that evokes the transformation of B cells to plasmocytes
is the interaction of mature helper T cell with the antigen determinant presented by the B cell.
Differentiation and proliferation of the B cell begin and the B cell is transformed into the
plasmocyte that synthesizes and secretes molecules of immunoglobulins, which are specific for
the given antigenic determinant.
B lymphocyte
PLASMOCYTE
receptor for
interleukin 4
complex pAg-MHC II
CD4
Mature helper T cell
interleukin 4
PK
PK
initiation of
interleukin 4 secretion
Interleukins are proteins secreted from interacting macrophages and immature helper T cells (IL-1
and 2) and from mature helper T cells during their interaction with antigen-presenting B cells (namely
IL-4, 2, and 6). They bind to specific types of interleukin receptors and affect significantly the blastic
transformation of both helper T cells and B cells.
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